Power system



Jan. 29, 1946. M. A. EDWARDS POWER SYSTEM Original Filed Dec. 19, 1942 2Sheets-Sheet 1 Inventor Martin A. Edwards, by W Hi Attorney.

Jan. 29, 1946. M. A. EDWARDS 2,393,619

POWER SYSTEM Original Filed Dec. 19, 1942 2 Sheets-Sheet 2 Fig. 5.-

F gs 3 :3 a is i 3 t E 2 Fig. 4.

Invent or":

Martin AEdwar'ds,

9 @Actorneg.

Patented 1.... 29, 1946 POWER SYSTEM Martin A. Edwards, Sootia, N. r.,lllilnol' to at... oral Electric Company, a corporation 0! New YorkOriginal application December is, 1542. Serial No. 469.538. Divided andthis application October s, 194:, Serial No. 505.431

1 Claims. (Cl. sac-1'1) My invention relates to power systems, and moreparticularly to speed responsive governing apparatus for gasorDiesel-electric power systems, especially of the type commonly used inself-propelled vehicles.

My invention described and claimed herein was previously described andclaimed in a. copending application, Serial No. 469,538, illed onDecember 19. 1942, as a Joint application of Donald E. Garr, John C.Aydelott, George M. Adams, and me and assigned to the same assignee asthe instant application. This application is a division of the saidJoint application, Serial No. 469,538. Certain features of the powersystem described but not claimed herein are claimed in said jointapplication. which application is being amended as aioint application 01Martin A. Edwards, Donald E. Carr and John C. Aydelott.

Diesel-electric power systems for self-propelled vehicles ordinarilycomprise a prime mover, such as a Diesel or'other internal combustionengine, arranged to drive a generator i'rom which power is supplied toone or more traction motors. The systems of this type, which are in moreor less common use at the present time, make use of traction motors ofthe series field type and utiline voltage control of the generator forcontrolling the torque and speed of the motors and reversing switchesfor controlling their direction of rotation. While such a system issuitable for application to vehicles in which all motors simultaneouslyundergo like changes of speed and torque, such as in Diesel-electriclocomotives. busscs, and the like, they are not sufliciently flexible tosupply the demands of certain other types of vehicles in which anoptimum of maneuverability is desired. This is especially true of suchvehicles as tractors, electric shovels, military tanks, and the likewhich rely upon the independent control of laterally spaced drivingelements for propulsion, steering, and braking. Essentially the sameProblems or propulsion, steering, and braking are encountered in twinscrew or paddle wheel boats, although these applications are simpliiledto some extent insofar as extreme rapidity of the response, while it maybe provided, is ordinarily not necessary.

For applications of the above nature, controlled current systems havebeen proposed. By such a system, I mean one in which the main generatoris connected in a series or loop circuit with the motor armatures andarranged to supply to the armatures a constant or otherwise continuouslycontrolled unidirectional current, while the excitation of the motorilcld windings is separately and independently controlled to determinethe speed, torque. and direction of rotation of each of the motors. Sucha system is shown in broad outline in British Patent 226,960 andsatisfactorily meets the demands of maneuverability out.- lined above.For example, it will be evident that both motors may he suddenlyreversed to efl'ect dynamic braking merely by reversing the relativelysmall energizing currents 01' their field windings while theunidirectional armature current is maintained constant or withinpredetermined limits. Similarly, the torque of any one motor alone maybe reduced or, in fact, reversed to effect steering operation.

While controlled current systems of the type shown in the British patentabove referred to are suitable and satisfactory for marine a plications,they have not heretofore been applied to land vehicles or to industrialapplications. in spite of their obvious advantages in respect tomaneuverability. The reason is evident when it is appreciated that,while the system of the British patent meets the principal demands ofmaneuverability, it is not characterized by rapid speed of responsesince such response is not ordinarily necessary in ship Pr pulsionapparatus. A land vehicle on the other hand, especially one which is tobe operated in close proximity to a number of other vehicles and overirregular terrain, must possess not only maneuverability. but also anextremely rapid rate of response.

My present invention relates to governing apparatus for gas-electricpower systems particularly arranged to enhance the speed of response ofthe system and to minimize overshooting or hunting. For convenience ofillustration. I am describing my governing system as applied to acontrolled current power system for a self-propelled vehicle of the typedescribed in aforesaid Joint application oi Edwards, Garr, Aydelott, andAdams. This particular electric vehicle propulsion system is beingillustrated as one suitable application of my invention because thissystem incorporates also a number or other features which increase thespeed of response of certain other parts of the system. These otherfeatures relate principally to motor and generator field excitation and,when combined with my new and improved governing apparatus. provide aunitary system particularly characterized by maneuverability.flexibility, and a remarkably rapid rate of response. I wish to have itunderstood, however, that my invention is not limited in its applicationto controlled current motor control systems. nor in fact to electricvehicle propulsion systems,

but is applicable in general to electric power systems comprising aprime mover and a generator driven thereby to supply power to electricmotors.

Accordingly, it is a general object o! my invention to provide animproved gas-electric power system which is simple, reliable, easy tooperate. and characterized by a rapid rate response.

It is a iurther object of my invention to provide a governing apparatusfor a. gas-electric power system which is characterized by a rapid rateof response and provided with improved damping means to preventobjectionable overshooting or hunting.

It is a still further object oi my invention to provide a new andimproved governing apparatus for an engine-driven generator arranged tocontrol both the engine and the generator to permit maximum utilizationof available engine power over a wide range of generator load currents.

It is a still further object of my invention to provide a gasorDiesel-electric power system for sell-propelled vehicles arranged topermit maximum utilization of available engine horsepower over a widerange of vehicle speeds, while permitting rapid and continuoustransition from motoring to electric braking operation at any speedwithout overloading any part of the system.

According to the illustrated embodiment oi my invention, the above andother objects are attained by providing a gasor Diesel-electric powersystem comprising a prime mover, a generator. and one or more tractionmotors, in which a unidirectional current oi continuously controlled anddefinitely limited magnitude is circulated through the motor armaturesand means are provided for separately controlling the excitation of themotor field windings independently to determine the speed, torque, anddirection oi rotation of the motors. By proper selection of forward andreverse torques for the separate motors, steering and forward or reversemotoring or electric braking operation may be effected without affectingthe direct control 01' the continuously circulating armature current.

I provide an engine driven generator having an output circuit includingthe armatures or all the traction motors connected in series circuitrelation. According to my invention, efllcient utilization is made ofthe maximum available engine horsepower over a wide range of vehiclespeeds by limiting the voltage-current characteristic of the generatorto the maximum available engine horsepower in the region between themaximum generator voltage, as determined by saturation, and maximumgenerating current. as determined by armature reaction, or by a suitablecurrent limit circuit. Each traction motor is also provided with aseparate engine-driven exciter provided with a manually controlledexcitation means to permit a smooth or continuous transition of motorexcitation from maximum forward excitation to maximum reverseexcitation. For any predetermined setting of the manually adjustablemotor field control means, the motor exciter field and, hence, the motorfield energization may be constant or may be connected to vary inaccordance with the motor current, motor voltage, or motor speed, or anycombination of these. In this system the line current is limited by anelectric valve spill-over circuit for reducing the generator excitationupon occurrence of the predetermined current, the spill-over value beingreduced as vehicle speed increases. Preterably, an electric valvespill-over circuit is also provided to reduce the power output of themotors whenever the vehicle attains a predetermined limiting speed.

For a more complete understanding o! my invention and a iurtherappreciation oi its objects and advantages, reference should now be hadto the following detailed specification taken in connection with theaccompanying drawings in which Fig. l is a schematic circuit diagram ofconnections for an electric power system embodying my invention; Fig. 2is a simplified circuit diagram of the current limit circuit: Fig. 3 isa simplified circuit diagram of the vehicle speed limit circuit, Fig. 4is a simplified schematic circuit diagram of the main motor fieldenergizing circuit; and Figs 5, 6 and 7 are graphical representations ofvarious operating characteristics of the system.

Referring now to the drawings, and particularly to Fig. 1, I haveillustrated schematically therein a Diesel or other gas-electric powersystem for a self-propelled vehicle. The system comprises a prime moverill of the internal combustion engine type, such as a Diesel engine,arranged to drive through a common shalt ii, a main generator l2, agenerator exciter l3, and a plurality of traction motor exciters II andi5. An output circuit from the armature of the generator I2 is completedthrough the armatures ot a pair of electric traction motors l6 and II.The output circuit 01' the generator i2 is a series or loop circuit andincludes a commutating pole winding l8 for the generator I! andcommutating pole windings l9 and 20 for the motors IG and I1,respectively. The generator output circuit may be permanently closed asshown or. if desired, suitable disconnecting switches may be included. Ihave also shown, permanently connected to the output circuit, a brakingresistor 2i arranged to be shunted during motoring operation by thecontact 22 of a braking contactor B. It will be understood, of course,that the representation oi the pair of traction motors l6 and I! ispurely diagrammatic and that each motor shown on the drawings mayrepresent either a single motor or a. group of motors arranged either inparallel or in series circuit relation. For pur- .poses oi.illustration, it may be assumed that each motor or motor group l6, I1 isarranged to drive the track on one side of. a track-laying vehicle, suchas a tractor, military tank, or the like. For example, the motor It maydrive the left-hand track, while the motor I! drives the right-handtrack.

It is well understood that an internal combustion engine 01' the typedescribed, when its speed is maintained substantially constant by agovernor, is capable of.delivering at its output substantially constantmaximum horsepower with the throttle fully open. As shown on thedrawings, an engine throttle lever 23 is controlled through a hydraulicthrottle linkage 24 from a governor 25 to maintain constant the speed ofthe engine ii, the setting oi the governor being manually controllableby means 01' a foot pedal 26 through a speed setting linkage 21 toselect a desired constant speed. By way 01' example, I have shown aflyball governor 25 arranged to rotate a governor shaft 28 through alever 29. The governor is biased to a normal collapsed position by atension spring 2911. Through a hydraulic connection 28a the shaft 28 isreleasably connected to an aligned shaft 281:. The connection 28:: iscoupled hydraulically to the engine lubricating oil system (not shown)and is arranged to connect the shafts 2e and 28b in substantially fixedpredetermined relation when oil pressure exists. when the oil pressureis zero, as when the engine is at standstill, the connection between theshafts 28 and 28b is effectively broken in the connector 28a. The shaft2311 has attached thereto an arm 30. Rotatably mounted upon the shaft38b is a U-shaped yoke 30a. The outer leg of the yoke 30a is normallybiased into engagement with the arm 30 by a tension spring 30b connectedtherebetween. The yoke 30a i connected by a rod 300 to the throttlelever 23. The throttle lever 23, rod 300, yoke 30a, arm 30 and shaft281), are all normally biased to the idling position of the lever 23 bya throttle spring 23a. This entire mechanism is so arranged that, upon adecrease in engine speed, the inward movement of the flyballs willrotate the shafts 28 and 28b in a clockwise direction, as shown on thedrawings. thereby to open the throttle and bring the engine speedsubstantially back to normal. The yoke 30a is also provided with aprojection 3| arranged to engage fixed stop 32 in the full open positionof the throttle thereby to prevent further movement of the throttle 23,but to permit overtravel of the lever 30 and a connected linkage for apurpose to be more fullydescribed hereinafter. The speed setting of thegovernor is controlled by a tension spring 33 connected between thelever 29 and a bell crank lever 34 in aiding relation with the spring290. The bell crank lever 34 is connected through a suitablelinkage,such as a hydraulic system 35. to the foot pedal 26 and is so arrangedthat depression of the foot pedal applies tension to the spring 33. Ihave also shown, connected to the speed setting bell crank lever 34through a lost motion linkage 35, a throttle solenoid 31. is will beexplained more fully hereinafter, the throttle solenoid 31 is arrangedto apply to the speed setting spring 33 an initial tension as soon asexcitation is applied to the field winding of the generator l2 therebyto raise the engine speed to a certain predetermined minimum runningspeed above idling speed. The lost motion of the link 36 permits furtherapplication of tension to the speed setting spring 33 through the footpedal 26.

Generator field excitation energization of the main generator fieldexcite ing winding 33 which is, in turn, controlled by the energizationof a plurality of control field windings 39, 43 and 4| on the exclterl3.

Preferably. the exciter I3 is a direct current generator or the typedescribed and claimed in Patent 2,227,992. issued to Ernst F. W. Alex-Martin A. Edwards on January 7, 1941. such a generator is of thearmature reaction excited type and is provided with a pair of currentsupply brushes 42, a series load compensating winding circuited brushesl3. Thecontrol field'windlugs 33, 43 and 4| are arranged to generate42a, and a pair of shorta voltage between the short-circulted brushes43, and the armature reaction 01 the current flowing through theshort-circuited connection as a result of this voltage sets up a flux insuch a direction as to provide an output voltage at the load brushes 42which is proportional to the excitation of the machine along itsshort-circuited axis. One of the desirable characteristics of anarmature reaction excited generator of this ype is that its outputvoltage responds very rapidly and with a high degree of amplification toany change in the energization of its control field winding. For thisreason, machines of this type are frequently referred to asdynamoelectric amplifiers.

Referring now more particularly to the control field windings of thegenerator exciter l3, the winding 33 is a stabilizing or antihuntwinding connected directly across the load terminals of the exciter inseries with a capacitor 44. The field winding 39 is thus excited onlyupon a change in exciter voltage and in a direction to oppose thechange. In main control of the generator exciter voltage is obtainedthrough the generator exciter forward field 4| and the diiferentialfield 40. Fundamentally. the forward control field 4| is excited from asubstantially constant voltage source of control power, such as abattery 45. while the diilerential field winding opposes the winding 4!and is connected in shunt to the commutating pole windings l3, l3, and20 in the generator output circuit for response in accordance with themain linecurrent. The excitation of the differential field winding 40 ismodified by certain auxiliary circuits which will be considered ingreater detail hereinafter. Specifically, the forward control fieldwinding 4| is energized across a governor controlled potentiometer 46which is connected across the ter- 40 minals of the battery through acircuit which may be followed from the positive terminal of the batterythrough a disconnecting switch 41, a suitable protective fuse 48. a wire13+. a normally open interlocked contact 49 on the braking contactor B,a resistor 50, the potentiometer 43, a resistor 5| and a ground wire 52which, in operation, is connected to the negative terminal of thebattery 45 through ground and one or the other of a pair of manuallyoperable ;dead man interlock contacts 53, 54 which will be more fullydescribed hereinafter. The control field winding 4| itself is connectedacross the potentiometer 46 through a pedal control rheostatll and afixed resistor 53.

In the drawings the various parts 0! the governor controlled mechanismare shown in their normal biased positions with the engine atstandstill. Under such conditions the fiyball mechanism 23 is completelycollapsed by the springs 23a and 33.

Since no lubricating oil pressure exists, the hydraulic connection 23ais broken, so that the throttle spring 23a holds the throttle 23 and thethrottle linkage 24 in the idling position. The overtravel spring 33!:holds the arm 33 and connected linkage in its position oi zero overtravel as shown in the drawings. When the englue is set into operationoil pressure is established in the connection 33a. thereby connectingthe spring 23a to oppose the spring 23a. The fiyballs assume theiridling position because of the throttle position. At this time thespring 33 ex- .erts no appreciable force and the opposing springs 29aand 33a are set to maintain the idling speed. for example about 650 R.P. M. As

soon as one "dead man interlock II or ll is closed, the throttlesolenoid I1 is picked up and sets the spring 83 to exert a predeterminedinitial tension in aiding relation to the spring 200, thereby toincrease the engine speed to a minimum running speed of, for example,1200 R. P. M. The engine speed may now be further increased bydepressing the pedal 26 to further increase the tension or the spring33.

It will now be observed that the potentiometer II is arranged to eflectno change in the voltage applied to the control field winding 4| duringmovement of the throttle from its idling posltion to its full openposition. If, however, when the throttle is in its full open positionthe engine is still unable to maintain the desired speed, the governorthrough the overtravel linkage connected to the arm 80 on the governorshaft 28b will continue to move the arm 30 and the slider of thepotentiometer 48, even though no further movement of the link 30a andthe throttle lever 23 is possible because of the stop 32. Suchovertravel of the governor will, through the potentiometer l6, reducethe voltage applied to the generator exciter Iorward field winding Ithereby to reduce the excitation oi the generator exciter I I and themain generator I! and to permit the engine to again come up to thedesired speed. By this arrangement, the power demand 01 theengine-driven generator I! is reduced to the middle range of itsvolt-ampere characteristic where it normally exceeds the maximumavailable horsepower output of the engine it, thereby to prevent slightengine stalling within this range and to permit maximum utilization ofall available engine horsepower over a wide range of vehicle speeds andgenerator load currents.

In governor overtravel operation, where the engine speed is controlledby change of the generator excitation in accordance with governorposition, it has been found that objectionable instability and huntingare encountered unless means are provided for rendering the generatorexcitation rapidly responsive to control by the governor and to preventovershooting of the control point or hunting." According to my invention, the governor overtravel field control system is renderedexceptionally stable by reason of the fact that the generator excitationis provided by the armature reaction exciter I3 which'incorporates theantihunt field exciting winding 38. As previously explained, theresponse of the armature reaction excited generator i3 is extremelyrapid, and large changes in its output may be produced by the control ofvery small amounts or input power. Accordingly, the governor overtravellinkage need control oi the-resistor liismovediromitsextremecounterclockwise position by acceleratingmovementofthepedalfl,therhe0stat llisgradually excluded from the circuitand the exciter excitation is built up as the engine speed increases.Preferably. the rheostat I! is arranged to be completely out out at arelatively low engine speed. For example, it as supposed. the engine isarranged to have a minimum running speed or 1200 R. P. M. with thethrottle solenoid Sl picked up, it has been found suitable to completelyout out the resistor it at a selected low en ine speed setting 0!approximately 1400 R. P. M. as has been indicated upon the drawings.Thus, it will be observed that for pedal positions beyond thatcorresponding to this selected low speed, but less than full open.neither the governor potentiometer 48 nor the rheostat it exerts anycontrol of the energization of the control field winding ll, so that theenergization oi the winding II is substantially constant within thisrange oi throttle positions. It will be understood that variation of the'field resistance 55 is not fundaonly very small control currents toproduce large and rapid changes in the power output of the exciter.Furthermore. the exciter operation is rendered stable, even under suchrapid changes of output, by provision of the stabilizing field excitingwinding 38, which is energized only upon mental, but that, if desired, afixed resistor may be substituted therefor. Beyond the full openposition oi the throttle. the governor 25 functions to reduce the powerdemand of the generator to equal the maximum available power output ofthe engine by control of the generator forward field ll through thepotentiometer it.

As previously mentioned, the control field winding 40 is a diilerentialwinding and is arrangedwhen energized to excite the generator exciterfield in opposition to the fiux oi the winding Ii. The field winding itis connected across the commutating pole windings l8, l9, and Ill in thegenerator output circuit .and is thus energized in accordance with mainline current, thereby to cause the output voltage of the generator I! todrop from a predetermined maximum value to a value Just suflicient tosupply the resistance drop 0! the output circuit when maximum current isflowing. The normal energization circuit of the winding 40 may befollowed (mm the positive terminal of the generator i2 through thewinding 40 and a current limiting resistor 51 to ground.

Line current limit For the purpose 0! limiting the current in thegenerator output circuit to a predetermined maximum value, supplementarymeans are provided (or abruptly increasing the energization of theexciter diiierential winding 40 when the line current attains apredetermined maximum value. The current limit circuit may be morereadily understood from the simplified diagram 0! Fig. 2. At Fig. 2, theconnection 0! the exciter difierential winding 40 across the commutatingpole windings l8, l9. and 20 in series with a resistor II is clearlyshown. To provide the maximum current limit, the resistor 51 is shuntedby a circuit including the battery It. a rectifier bridge 58. and anormally open interlock contact SI of the braking contactor B. Aspointed out heretofore, the braking contactor B is picked up duroveroperation takes place is battery is negative, and the side of theresistor connected through the rectifier bridge to the positive side ofthe battery is positive, so that the voltage drop across the resistor 51opposes the voltage of the battery through the blocking rectifier bridge58. the rectiflers preventing any flow of current through the loopcircuit from the battery. It will now be evident that when the currentflowing in the output circuit 01 the generator l2, and hence the currentflowing through the exciter diil'erential winding II) and the resister51, attains such a value that the voltage drop across the resistor isgreater than the voltage of the battery 45 by an amount sufllcient tobreak down the rectitlers in the arms of the bridge 58, current willspill over from the positive terminal of the resistor 57 through thebattery 45 thereby to provide a current path parallel to the resistor I!and in series with the dii" i'erential control field lll. By thusintroducing a very low resistance shunt connection around the resistor51, the net resistance of the circuit through the control field winding40 is reduced and the energlzation of the control field winding 40 issharply increased, thereby to diminish the excitation of the generatorexciter l3 and to cut down the excitation of the main generator II. Themain line current at which the above spillthe maximum cur-' rent limitfor the generator output circuit. The added energization thus suppliedto the control field winding 40 is suiilciently great so that no furtherincrease in the main line current is possible after operation of thecurrent limit circuit.

Viewed in another way, the current limit breakdown of the rectifierbridge 58 ties the negative terminal oi the control field winding 40 tothe positive terminal of the battery 45 through a circuit or very lowresistance. In this way the potential of the nega ive terminal of thewinding 40 is limited to ba tery potential and the potential oi thepositive terminal of the gen erator I! is limited to battery potentialplus the slight voltage drop through the winding Ill. Any tendency ofthe potential oi the negative terminal oi the generator I 2 to increasepositively above this fixed potential, as by increased current throughthe resistor 51. causes such a large flow or current through thedifferential winding 40 and the battery spillover circuit that thegenerator excitation is immediately reduced to maintain the limitedmaximum current.

It is desirable to reduce the maximum current limit as the vehicle speedincreases, that is, the current limit circuit should be brought intooperation at a somewhat lower line current when the vehicle speed ishigh than when the vehicle speed is low; This condition is imposed bythe limits of good commutation by the traction motors i8 and I1. It iswell understood that the maximum current which may be satisfactorilycommutated at high speed is less than that which may be satisfactorilycommutated by the same machine 'at a lower speed. For this purpose, Iprovide a tachometer generator 80 having a separately excited fieldwinding 8| energized directly from the battery 45 through the wire 3+.The tachometer generator 80 is driven from the shaft of one or thetraction motors, for example the traction motor ii, and thus provides anoutput voltage proportional to vehicle speed for modifying the operationof the current limit circuit. From the preceding discussion of thecurrent limit circuit itself. it will be recalled that the shunt circuitacross the to diagonally opposite bridge 58. These points 62 and 63 atFigs. 1 and tlcularly to resistor 51 is connected points of therectifier have been identified as 2. Referring now par- Fig. 2, it willbe observed that a potentiometer 84 having a variable voltage tap 65 isconnected directly minals of the tachometer generator to the other 68and and 62 of the across the output ter- Bll to supply pair ofdiagonally opposite points 81 oi the rectifier bridge voltageproportional to vehicle speed. rectifying action of the bridge 58, this58 a suitable Due to the speed voltappearing permanently bridgeregardless of the direction of motion of the vehicle, the point 63 beingpositive with respect to the point 62. From Fig. 2 it may also beobserved that in the loop circuit comprising the battery 45, the

rectifier bridge 58, and t he resistor 51. the voltage drop across theresistor 51 and the speed voltage across the points 62 and 83 of therectiher bridge 58 are connected additively and in opposing relation tothe voltage of the battery l5. Thus, for current limit operation, thetotal voltage available for bu age to efl'ect a spill over citing thebattery volt-- of current through the rectifier bridge and the batteryfrom the positive terminal of the resistor portion to the speed of agebetween the points I! is increased in prothe vehicle by the volt- 82 and68 of the rectifier bridge. Since the battery voltage is substantiallyconstant, it will be clear that as the vehicle speed increases spillover through the battery and. hence, current limit operation will takeplace with progressively smaller voltage drops across the resistor 51,that is,

at reduced main line current. since the voltage drop across the resister51 is proportional to the main line current. In this way the maximumcurrent limit is reduced as the vehicle speed increases.

Traction motor field control Referring now to the field control for thetraction motors l8 and that each motor is rately excited field II, itmay first be noted provided with a, main sepawinding, the motor l8having a winding 18 connected to the output terminals or thedynamoelectric exciter H and the motor I! having a field winding Hconnected to the output terminals of the dynamoelectric exciter l8.Preferably, the exciters II and ii are machines oi the armature reactionexcited type heretofore described in connection with the generatorexciter l3 and include series load compensating windings 10a and Ho.citers H and ii are provi trol field respectively. The exded also withmain conwindings I2 and II, respectively. which are energized throughmanually controllable potentiometer bridges 14 and II, respectively, inaccordance withthe voltage drop across the commutating pole windings i8,i9

generator output circuit. for the windings 12 and the positive terminalof erator I! through a resistor the points Ill and BI bridges II and 15,respectively,

and 20 of the main The energizing circuit 13 may be traced from the l8and a wire 11 to the potentiometer and hence through both sides of bothpotentiometers in parallel circuit relation to a grounded wire 82. Thus,the

voltage appearing across each potentiometer" and Ii from the pointsproportional to the current put circuit as indicated by oi the generatoroutthe voltage drop across the commutating pole windings I8, I 9 and 20.A

suitable voltage for appli cation to the motor exand II to ground isciter field windings I 2 and II is selected on the potentiometers H and15 by means of manually positionable steering handles 83 and 84,respectively.

It is desirable at this point to note certain mechanical features of thesteering handles 83 and i4. Primarily, these handles have for theirpurpose the determination of the standard of energization of the motorfield windings It and H by selection from the potentiometers I4 and 15of a suitable voltage proportional to line current for application tothe motor exciter field windings l2 and It. It will also be noted thatthe dead man" interlock switches 83 and 54, previously mentioned, areactuated by handle levers attached to the steering handles and thatthese switches are arranged to be closed as soon as the handles aregrasped by an operator. Preterably, as indicated in the drawings, thesteering handles 83 and ill are biased to a normal position such thatsubstantially full forward voltage is applied to the control fieldwindings I2 and 13. Ii desired, the steering handles may also bearranged to actuate a number of auxiliary switches and rheostats toefiect refinements oi the operation described and claimed in theaforementioned copending application Serial No. 469,538. and in aconcurrently filed application, Serial No. 505,468, filed by George M.Adams and John C. Aydelott and assigned to the same assignee as theinstant application. Since these i'eaturesare no part of the presentinvention. however, they have been omitted to simplify the description.

It will be clear from the above description of the potentiometers 14 andI that, with the steering handles 83 and 84 set in any predeterminedposition, the energization of the field windings I2 and ll of the motorexciters I4 and I! will vary in accordance with the magnitude of themain line current. Furthermore, since the exciters ll and I! serve asdynamoelectric amplifiers, it will be clear that the ener gization ofthe motor field windings Ill and 1! is proportional to the armaturecurrent or the motors and that, except as modified under certainconditions explained hereinafter, the excitation characteristic of thetraction motors it and I1 is similar to that or a series motor. It willbe understood that the steering handles 83 and 84 are independentlycontrollable, so that the standard of excitation of each motor may beselected independently and set at any desired point be tween fullforward field and full reverse*'field. The energizing circuit for themotor exciter windings I2 and I3 is shown in simplified form at Fig. 4.While Fig. 4 shows only a single potentiometer bridge, it will beunderstood that the potentiometers ll and are connected in parallelcircuit relation The series excitation characteristic of the tractionmotors I8 and I1 is desirable because it increases the speed up to whichmaximum utilization of available engine horsepower may be obtained.However, it has been found that even higher motor speeds than may bethus obtained are desirable at the point of maximum utilization 0!engine power. This demand arises in part from the fact that saturationof the motor fields at low vehicle speeds in an eifort to obtain maximumtractive effort for starting impairs to some extent the proportionalitybetwee'n line current and the motor flux. Accordingly. in order furtherto increase the motor speed up to which maximum utilization of enginehorsepower may be main control l5, respectively.

obtained, 1 provide additional means for exaggerating the seriesexcitation characteristic of the motors by controlling the motorexcitation in inverse proportion to vehicle speed, as well as in directproportion to the main line current. For this purpose, I utilize thevoltage oi the tachometer generator 60 to energize in proportion tovehicle speed a pair of differential control field windings 50 and St onthe motor exciters i4 and Referring to Figs. 1 and 3, it will beobserved that the control field windings 9B and 9! are connected inseries circuit relation across intermediate points 92 and 93 of a bridgecircuit, the line terminals 94 and 95 of which are connected to theterminals of the tachometer generator Bil. Fig. 3 clearly shows thebridge circuit which comprises a pair of resistors 98 and 91 connectedin series circuit relation between the terminals 94 and 95 and aresistor 98 connected in series with a rectifier 99, also between theterminals 94 and 95. The rectifier 99 is connected normally to conduct,the terminal 91 being positive. The arms of the bridge are soproportioned that the point 92 is normally positive with respect to thepoint 93 and the motor exciter field windings and M are connectedbetween these points. The energizing circuit for the windings 90 and illmay be traced from the bridge terminal 92 through a rectifier "In andthe control field windings Bil and BI in series to the terminal 93.Thus, as the vehicle speed and the tachometer voltage rise, the voltageacross the bridge points 92, 9! increases proportionately. As indicatedat Fig. l, the difi'erential control field windings 9B and Si areenergized in opposition to the main field windings I2 and 13,respectively, so that as the energization of the windings 8i! and SIincreases with increasing vehicle speed, the net excitation of thetraction motor exciters ii and i5 and, hence, the excitation of thetraction motors l6 and I1 themselves is reduced in proportion with thespeed of the vehicle. It will be borne in mind that this eflect issuperposed upon the reduction of traction motor excitation due to thedecrease in main line current as the vehicle speed increases and, hence,produces an exaggerated series motor characteristic.

The reason for making use oi' th rectifier 99 as one arm of the bridgecircuit of Fig. 3 will become evident from an examination of the curvesof Figs. 6 and '1. The rectifier 59 serves as a nonlinear resistor whichdecreases its resistance as the voltage across the bridge increases.Referring first to Fig. 6, I have here shown the relation between thevehicle speed and line current from zero speed up to maximum vehiclespeed. The maximum vehicle speed is indicated at Fig. 6 by a line lfll.The maximum vehicle speed is indicated at Fig. 6 by a line III. whichthe vehicle speed is limited to this maximum will be describedhereinafter, but for the present it is suflicient to note that thebroken line curve I02 of Fig. 6 indicates the manner in which the linecurrent would diminish as the vehicle speed increases were it not forthe additional iield weakening due to the energization oi the motorexciter field windings 90 and SI. Also, at Fig. 6, the straight line I03indicates the maximum current limit which, as has been heretotoreexplained, diminishes as the vehicle speed increases. As described inthe preceding paragraph, the excitation of the control field windings SIand BI increases with vehicle speed and this increase is substantiallylinear over a portion curve I02. As indicated at Fig.

ings through the rectifier 80 upon ill of the curve of Fig. I so thatthe line current of Fig. 6 follows a curve I rather than the 6, thedeparture of the curve ill! from the curve I02 becomes more pronouncedas the energization of the field windings 00 and 9!, as shown in Fig. 7,increases. Thus, due to the efiect of the field windings 00 and BI, theline current begins to show an increase at a portion lliia oi the curveI00 and begins to approach the current limit value. Since it isundesirable that the current limit be exceeded at this point, furtherincrease in the energization of the field windings 00 and SI isprevented and, in fact, a net decrease in their energization may beproduced by the action or the rectifier 90. The negative resistancevoltage characteristic of the rectifier 00 becomes pronounced atrelatively high tachometer voltages thereby to reduce the resistance ofthe bridge arm in which it lies and to preclude further increase in thevoltage of bridge terminal 92 with respect to terminal 93. Indeed, theeii'ect of the rectifier 90 may be made o pronounced as to actuallyreduce the voltage across the control field windings 00 and 0! despitefurther increases of tachometer voltage (see the portion [04a of thecurve I04, Fig. '7). As indicated at Fig. 6, then, this has the efl'ectof again reducing the line current at a portion [05b of the curve I05.By thus limiting and, in fact, reducing the energization of the windings00 and 0! in the high speed range, an increase of the line current tothe limiting value is avoided. The non-linear bridge circuit motor fieldcontrol systems described above are claimed in an application of GeorgeM. Adams. Serial No. 505,469, filed concurrently herewith and assignedto the same assignee as the instant application.

Vehicle speed limit In addition to their Motion of exaggerating theseries excitation characteristic of the traction motors, the motorexciter control field windings 00 and 0| are connected in circuit withthe control battery 45 to provide a vehicle speed limit control andindication. For this purpose, the voltage of the tachometer generator 60is matched against the voltage of the battery 45 through a loop orspill-over circuit including the control field windings 00 and 0| of themotor exciters II and II and a pair of blocking rectifiers I00 and "8a(see Figs. 1 and 3), the rectifier i06a being in the rectifier bridge08. The blocking rectifiers normally prevent a flow of current from thebattery through the tachometer generator 30 and the field windings 90and 8|, but permit a spillover current from the tachometer generatorthrough the battery and the control field windings 00 and ii to supplysupplementary excitation to the field windings whenever the vehiclespeed is such that the tachometer voltage, or a suitable portionthereof. exceeds the battery voltage. The spill-over connection may betraced at Figs. 1 and 3 from the positive terminal of the tachometergenerator 00 through a wire I 01, the rectifier "la. the wire 13+, andthe battery 45 to ground, and from ground through the rectifierspill-over in speed limit operation. From the above circuit it will beevident that the rectifiers I06 and [00a are so arranged that currentcannot fiow from the battery 45 through the tachometer generator 00. Onthe other hand, when the vehicle attains such a speed that the voltageat" the output terspill over from the l of the curve of Fig.

I 00. a suitable visual or audible speed indicating device I00, themotor exciter control field windings II and 0|, and the resistor 07 tothe negative terminal of the tachometer generator 00. It will now beclear that the rectifier I00 is included in the normal or bridgeexcitation circuit or the control field windings 90 and 0| for thepurpose of preventing short-circuiting of these field windminals of thetachometer generator attains a value greater than that 01' the batteryvoltage, the two blocking rectifiers will, rmit current to tachometergenerator through the battery, the overspeed indicating device I00, andthe motor exciter control field windings 90 and 9|. Furthermore. it isto be noted that the vehicle speed at which such spill over will occuris substantially unaffected by changes in the battery voltage because ofthe fact that the tachometer generator is battery excited, so that asthe battery voltage increases or decreases, the tachometer generatorvoltage increases or decreases proportionately.

The operation or the speed limit circuit may now be followed. It will berecalled that, independently 01' any energization of the motor excitercontrol field windings 90 and 0| through the speed limit spill-overcircuit traced in the preceding paragraph, these field windings arecontinuously excited through the bridge circuit 01' Fig. 3 in the mannerindicated at Fig. 7. Referring now to Fig. 6, as the vehicle speed limitis approached, the line current is in the region I051: of the curve I05.If now, the vehicle speed becomes such that the tachometer voltagecauses current to spill over through the speed limit circuit thereby toenergize the overspeed indicating device I08 and to superpose additionalenergization upon the normal energization oi the motor exciter fieldwindings and iii, the total energization of the windings and 9| willincrease sharply as indicated at the portion [04b 7. Such increase inthe en'- ergization of the differential windings SI and 9' will cause asharp decrease in the net motor exciter excitation and, hence, in thenet motor excitation, so that th line current will increase sharply asindicated at IBM: of Fig. 6. i This sharp increase in the line currentwill bring the line current to the limiting value at the speed ofoperation as indicated by the curve I03 of Fig 6, so that the linecurrent limit circuit previously described will be brought intooperation. With the motor current thus limited at the'the current limitvalue, the motor power is determined by the motor voltage. The motorvoltage is proportional to the motor speed and to the excitation of themotor field windings, so that with the motor speed remaining at themaximum value and the motor excitation severely limited by the increasedexcitation of the motor exciter fields 90 and 9|, the motor power willdrop ofi thereby to reduce the vehicle speed. When the vehicle speed isthus reduced, the tachometer voltage is reduced below the spill-overvalue and supplementary excitation of the motor exciter fields 00 and SIceases. It will thus be clear that the system will regulate on thispoint to maintain the vehicle speed at its maximum value.

The above operation of the system under speed limit conditions will beclarified by a consideration of Fig. 5. This figure is a more or lessconventional diagrammatic representation of the power characteristics 01a Diesel-electric system. Thus, the main generator l2, if driven at anyselected speed, will exhibit an-inherent full load volt-amperecharacteristic of the general shape oi the curve A. while the maximumpower output 01 the engine II will be substantially constant, asindicated by the curve B. Ordinarily, the oapacities of the variouselements of the system are so selected that these curves intersect inthe central operating range in order to attain maximum utilization ofthe available engine power over the widest possible range of vehiclespeed. From the curves A and B, it will be evident that within thecentral portion of the operating range the generator is demanding morepower than the engine is capable of supplying and that the only way thatthe curves may be caused to coincide, as they must, is for the poweroutput of the generator to be slightly diminished by slight enginestalling within this range. Since such reduction in the engine speedalso cuts down the available power output from the engine, it isdesirable, if possible. to cut down the generator demand so that it justequals the engine output. According to my invention, provision is madefor reducing the generator excitation by means of the governor operatedpotentiometer I6, previously described. By way of example, the curves Aand B have been drawn for the full open position of the engine throttle.With the throttle in this position, the potentiometer 46 has eilected nochange in the potential applied to the generator exciter field ll.However. as previously explained, the governor is provided with acertain degree of overtravel so that, it with the throttle in the fullopen position the engine speed still cannot be maintained, the lever 25attached to the governor moves the slider of the potentiometer N toefiect a reduction in the energization of the without further opening ofthe engine throttle. By thus reducing the excitation of the maingenerator II, the power demand of the generator is reduced to a pointwhere it just equals the available power output of the engine. Thisaction is of a regulatory nature and results in bringing the generatorvolt-ampere characteristic into coincidence with the available poweroutput the engine over the constant power range C of the resultantgenerator volt-ampere characteristic shown in solid lines at Fig. 5. Itwill be understood that in the region D, the voltage of the generator islimited to a predetermined maximum value by saturation of the generatorfield poles. The portion E on the curve of Fig. is determined byoperation of the current limit circuit at zero or low vehicle speed. Itwill be understood that as the vehicle speed increases, the currentlimit is reduced as indicated by the constant current lines E1 and E: ofFig. 5. The portion F of the generator volt-ampere characteristicrepresents the pure resistance drop through the armatures oi thetraction motors along which the line current built up in proportion togenerator voltage when the vehicle starts from standstill.

Referring again to the operation of the vehicle speed limit with thecurve of Fig. 5 in mind. it will be evident that, if the vehicle isoperating at a point I III on the generator volt-ampere characteristicwhen the vehicle speed reaches its maximum value, the sharp increase inmotor exciter diflerential field excitation produced by the speed limitcircuit. and indicated at Fig. '7, and the consequent sharp increase inline current indicated at Fig. 6 will cause the operating point ill! ofFig. 5 to move down the curve C to the maximum speed current limit curveE2, and hence down the current limit curve E: to a point such generatorexciter field winding ll aseaeio as Ill or Fig. 5. Itwillbc clearthatthe system power represented at the point III is less than thatrepresented by the constant power curve C, so that the power output ofthe motors will be reduced and the vehicle will slow down as previouslydescribed.

Stabilization of motor excitation It will be understood from the abovethat when the vehicle is moving iorward under power, thecounter-electromotive force of the motors is almost equal and oppositeto the output voltage of the engine driven generator l2. Since the motorvoltage is proportional to the product of the motor flux and motorspeed, it is apparent that when the vehicle is traveling at a relativelyhigh speed the motor flux is quite weak, while the generator 12 isoperating under partial saturation in its upper range of voltage. Undersuch conditions it will be evident that the motor exciters l4 and I5 arecapable of forcing an extremely rapid change 01' motor field flux withonly a. relatively small change of motor field energization, whilebecause of the generator field saturation a relatively large voltagechange at the generator exciter I3 is necessary to produce aproportional change in the generator field flux. Accordingly, if theexcitation of the traction motors is suddenly reduced or reversed toinitiate dynamic braking opera tion. it will be clear that a transientcurrent of considerable magnitude might flow in i 2 generator outputcircuit because of the fact that the motor field flux will reverse muchmore quickly than the generator flux can be reduced by operation of thecurrent limit circuit. For the purpose of bringing this undesirablecondition within permissible limits, the motor exciters II and ii areprovided with stabilizing control field windings H5 and H6.respectively. Referring to the drawinss, it will be observed that thepositive terminal o! the motor exciter l5 and the negative terminal ofthe motor exclter II are connected together and to ground through a wireI", while the positive terminal of the motor exciter I4 is connectedthrough the stabilizing control field winding iii, a condenser Hi, andthe stabilizing control field winding I IE to the negative terminal ofthe motor exciter II. Thus. when the system is operatin under steadystate conditions, no current flows through the stabilizing windings Illand H5, the condenser Ill merely being charged to a potential equal tothe sum of the potentials across the output terminals of the motorexciters II and IS. The stabilizing circuit is a loop circuit comprisingthe armatures oi the motor exciters I4 and 5, the control field windingsl I! and H6, and the condenser "8, all in series circuit relation, thecircuit being tied to ground between the armatures of the motor excitersby' the wire I".

It may now be observed that, if the voltages across the output terminalsof both motor exciters are changing in the same sense, that is. itincreased or decreased motoring or braking torque is simultaneouslybeing called for on both exciters or ii the excitation of both excltersis suddenly and simultaneously changed from forward excitation toreverse excitation to shift from motoring to braking operation, theondenser I I! is charged or discharged (depending upon the direction orthe change) through the stabilizing control fleld windings H5 and H6.These stabilizing windings are so arranged that the components of fieldexcitation which they produce under Such conditions of change in themotor exciters I4 and i5 tend to oppose the change of aaaacic voltage ineach machine. In this manner the rate or change oi output voltage or themotor exciters is suillciently decreased so that the genorator exciterdiflerentialfleld Ill-is able to make the flux of the generator field I8follow the motor fiux sufilciently closely to prevent an exceedinglyhigh transient current during a rapid change of motor excitation. It, onthe other hand, the voltage or only one 01' the motor exeiters ischanged either by increasing or decreasing the field excitation of thatmotor, while the voltage of the other motor exciter is maintainedconstant, the condenser'lll will also undergo a change in charge. Inthis case, however, the current throughthe stabilizing field windings H5and I II will be in the same direction in both windings, so that thecurrent through the stabilizing winding of that motor exciter whosevoltage is undergoing change will tend to oppose the change, while thecurrent through the stabilizing winding of that motor exciter whosefield excitation is not otherwise undergoing change will tend to producea change of voltage of that exciter in a. direction opposite to thevoltage change in the first exciter. This latter eflect is desirable inconnection with steering operation. Steering is efiected by moving onesteering handle to change the torque of the associated motor. In orderthat maximum steering efllciency may be attained, means may be providedfor automatically changing the torque of the other motor in the oppositedirection without necessitating movement of the other steering handle.Such means are described and claimed in the aforementioned copendingapplication oi Edwards, Garr, Aydelott, and Adams. It will be clear thatthe above transient efi'ect aids such an automatic change in motorexcitation.

The stabilizing arrangement described above is described and claimed inthe aforementioned concurrently filed application oi George M. Adams,Serial No. 506.469.

Steering As briefly mentioned in the foregoing paragraph, steering ofthe vehicle is eflected by actuating one or the other of the steeringhandles to reduce or reverse the torque of the associated motor withoutreducing the torque of the other motor, thereby to cause the vehicle toturn towards the side upon which the torque is reduced or reversed.Referring to Fig. 1, if the steering handles are in the full torwardmotoring position shown, steering may be eiIected bypulling one handle.for example, the right-hand steering handle at backwards toward or tothe null position. It very abrupt steering is. required, the handle maybe pulled back into the reverse quadrant to eilect reversal of thetorque and braking operation of the right-hand motor. During suchoperation, the left-hand motor continues to exert a forward motoringtorque. It will of course be understood that if the vehicle is travelingin the reverse direction. steering is eflected by moving one handletoward or into the forward quadrant.

Dimamic braking operation Dynamic braking operation without steering iseiIected by drawing both steering handles 83 and 84 simultaneously iromtheir forward quadrants into their reverse quadrants. By this operationthe field excitation of the traction motors l6 and I1 is reversed sothat these motors act as generators, their voltages adding to thevoltage or the main engine'driven generator l2. Under these conditions,the series excitation characteristic of the traction motors becomes aseries generator characteristic having a marked cumulative eiiect; thatis, as the excitation oi. the traction motors is increased in a brakingsense, the line current is increased, and as the line current increases,the excitation of the traction motors is further increased. This eifectis aggravated by the tendency to transient overshooting oi the linecurrent limit previously explained as resulting from the inability Ofthe generator to reduce its voltage as quickly as the motor voltage isreduced when making a sudden transfer to dynamic braking operation.

While the exciter stabilizer windings l5 and [6 limit the above currentlimit overshooting to permissible values, it is also desirable to limitthe braking torque when such high line currents exist. Indeed, it hasbeen found desirable to limit the braking torque at high line currentseven though the line current does not exceed the current limit value.For this purpose, means are provided for limiting the excitation of thetraction motors l5 and H to a, predetermined maximum value so that theseries excitation characteristic of the traction motors is transferredto a constant or shunt excitation characteristic upon the occurrence ofa predetermined high line current. Fig. 4 shows the manner in which theenergization of the main motor exciter control field windings l2 and i3is limited to a. definite maximum value. As has been previously pointedout, Fig. 4 shows the potentiometer bridge 14 (the potentiometer bridge15 being connected in parallel circuit relation therewith) connectedacross the inter-pole windings l8, l9, and 20 in .the generator outputcircuit in series with the resistor 16. Furthermore, it will now beunderstood that the basic series characteristic of the motor exciterwinding 12 arises from the fact 4|) that the potential at the point 80of the potentiometer H is proportional to the voltage drop across thecommutating pole windings i8, i9, and 20 as derived through the resistor16. In order to limit the potential at the point 80 to a predeterminedmaximum value, this point is tied through a blocking rectifier I26 to apoint of intermediate voltage I21 on the battery 45. From Fig. 1, itwill be evident that the point I21 has a fixed potential slightly aboveground potential as determined by the battery 45. So long as thepotential or the point 80 remains below that of the point III on thebattery 45, current cannot flow from the battery through either thepotentiometers or the commutating pole windings I8, I 9, and 20 becauseof the presence of the blocking rectifier I26. However, should the linecurrent through the commutating pole windings l8, l9, and 20 attain sucha value that the potential at the point 80 tends to attain a value inexcess of the potential at the point I21, current will spill over fromthe point to through the rectifier I26 and the connected portion of thebattery 45 to ground thereby limiting the potential of the point 80 to apredetermined maximum potential. After this operation, the potential ofthe point 80 remains substantially fixed regardless of the value towhich the line current, and hence the potential drop across thecommutating pole windings I8, [8, and 20, may go. Therefore, when theline current attains a value sufiicient to cause such spill over, theenergization of the motor exciter control field windings 12 and I3 and,accordingly, the excitation of the traction motors i6 and i1, attains afixed maximum value and is possessed of a shunt characteristic.

In this manner, the undesirable cumulative eii'ects of motor fieldstrength are minimized when going into dynamic braking operation. Itshould be noted that the excessive potential at the point Bil may not bedue alone to a simple resistance drop across the commutating polewindings II, I9, and 28. Upon a very rapid change in line current, asdue to a quick reversal of the traction motor field windings, such anexcessive potential may arise at least in part from' the inductive dropacross the commutating pole windings. The spill over circuit through therectifler lli guards also against such excessive inductive transientpotentials.

Under conditions or rapidly changing line current in going suddenly intobraking operation as described above. the inductive potential appearingacross the interpole field windings I8, I9. and Ill is utilized tohasten the decrease in the excitation of the generator I2 to its steadystate value. Ffor this purpose. the energiaation oi the differentialileld winding ill or the generator cxciter i3 is arranged to go toexceedingly high values. It will be recalled that the diflerentlal fieldwinding 40 of the generator exciter II is energized across thecommutatingpole windings l8. I8, and 20, so that any inductive voltagedrop appearing at the positive terminal of the generator I2 will providea transient increase in energization of the winding 40 and, hence. arapid decrease in the net excitation of the generator exciter I3 and thegenerator I2. By forcing an increased current through the diflerentialfield winding 40. the inductive voltage at the positive terminal of thegenerator I2 also appears across the resistor 51 in the generatorexciter diflerential excitation circuit and tends to bring intooperation the line current limit circuit previously described. so thatthe energization of the generator exciter differential control field I.may be further increased by spill over of the current limit circuitthrough the rectifier bridge 58 and the battery 45 as previouslydescribed.

Operation In view oi the foregoing detailed explanation o! the variousparts of the system and their manner of connection and mode ofoperation, the operation of the system as a whole will readily beunderstood irom the following brief description from the viewpoint ofthe operator.

Three simple and convenient controls are provided at the driver'sposition. namely, the-accelerator pedal 28 for controlling the speed andpower output of the internal combustion en ine. and the left and rightsteering handles I! and 84. respectively, for independently controllingthe speed, torque. and direction of rotation of the leftand right-handtracks. As previously described, the steering handles 83 and 84 areprovided with dead man" interlocks 53 and it normally biased to an openposition.

Starting of the engine It has been described heretofore under thesection on "Generator field excitation.

Motoring operation may now be initiated simply by grasping one or bothof the steering handles to close one or both of the "dead man"interlocks 53. 54. The 'interlocks 53. 54 are connected in parallelcircuit relation. and the closure of either interlock completes anenergizing circuit for the main control field winding II of thegenerator exciter I! through a circuit which has been previously traced.Simultaneously, an energizing circuit is completed from the positiveterminal of the battery I! through an actuating winding I" of thethrottle solenoid 81 to the wire 52 and through the dead man" interlocks53 and it to ground. This circuit also includes a normally closedinterlock contact III on an overspeed device III. The overspeed deviceI3! is arranged to be actuated upon overspeeding of the engine III todeenergize the throttle solenoid 31. As has been explained hereinbefore,the actuation of the throttle solenoid Sl adjusts the setting of thegovernor to maintain a. predetermined minimum running speed of theengine III thereby to assure adequate power for steering or braking eventhough the accelerating pedal 26 should be accidentally released.

Closure of one oi the "dead man interlocks 53. 54 to initiate motoringoperation also completes an energizing circuit through a reverse powerrelay FPR for an actuating coil I of the braking eontactor B. Thebraking eontactor B is normally open. but is closed during motoringoperation to shunt the braking resistor 2| in the main generator outputcircuit. The reverse power relay R-PR for an actuating coil I35 of thebraking I" and a directional winding I31. Upon closure of one of the"dead man" interdlocirs 53, N. on energizing circuit iscompieted for thepolarizing winding I36 from the wire B+ through a normaiiy closedinterlock contact I38 on the braking contactorBandthewinding llltot'ewlreii and hence to ground. When so enei ized. the reverse power relayRPR. picks up and is retained in its actuated position by a holdingcurrent of limited value through a resistor I39 which shunts thenormally closed interlock contact I38.

Upon actuation of the relay RPR. an energizing circuit is completed forthe actuating winding I35 0! the braking eontactor B through a circuitwhich may be traced from the wire 13-]- through a normally openinterlock contact I on the reverse power relay RPR, the actuatingwinding II! of the braking eontactor B, a normally closed interlockcontact Ill on the braking eontactor B. and a normally closed interlockcontact I42 on a time delay relay TDR to ground. Thus energized. thebraking eontactor 13 picks up to close the shunting contact 22 aroundthe braking resistor 2| and to complete for its actuating coil I35 aholding circuit through a current limit resistor It! and a self-actuatednormally open interlock contact I. After the eontactor B picks up, itcloses a circuit for an actuating coil I 01 the time delay relay 'I'DRthrough the interlock contact 49 on the eontactor B. Opening of the timedelay relay contact III has no eflect for .the present because it is nowshunted by the lock-in contact I on the eontactor B.

By way of summary then. simple closure of one or the other of the "deadman" interlock contacts 53 or 54 immediately increase, the engine speedto a predetermined minimum. picks up the braking eontactor B to shuntthe braking resistor 2|. and completes the field excitation circuit forthe generator exciter II. The consequent current flowing in. the outputcircuit of the generator I2 produces a voltage drop across thecommutating field windings I8. I! and 20 and thus energizes the maincontrol field windings I2 and 13 of the traction motor exclters II andI5 through the potentiometer bridges II and I5. respectively,

so that a torque is supplied to the driving wheels. Since the steeringhandles 8! and Ill are in their positions oi'maximum forward torque, thevehicle will begin to accelerate from standstill in the forwarddirection. As the vehicle comes up asoaoro to speed, the motor currentfalls ofi'and the voltagerisestromapoint I30 onthemlinel 'oi Fig. andalong a low engine speed power curve C to some point, such as the pointill. It will be understood that, as the line current diminishes, themotor torque also diminishes so that the point III is determined as thatpoint at which the total motor torque just overcomes the resistance tomotion of the vehicle.

With the establishment or current in the generator output circuit, thedirectional winding I81 01 the reverse power relay RPR is energized inproportion to the motor voltage and cumulatively with respect to thepolarlllng winding I36 0! the reverse power relay. The winding Ill thutends to hold the reverse power relay inits actuated position.

To increase the speed of the vehicle the acceierator pedals 26 must bedepressed. The

engine it is thereby speeded up and its power increased, thus increasingthe generator output current and the motor torque. The vehicle will,therefore, speed up until the increased motor torque just balances theresistance of the terrain over which the vehicle is operating. when thepedal 26 is fully depressed operation will be along the curve C of Fig,5, as at the point H0. It should be noted that the speed of the vehiclecannot'be increased simply by moving the steering handles Iorward(assuming that they had previously been moved from the maximum forwardposition) to increase'the motor excitation. This will be evident fromthe fact that no greater power isthereby supplied to the motor, sincethe generator power output is not changed by moving the steeringhandles. The only effect or moving the steering handles forwardsimultaneously is to increase the motor voltage and, thus,

decrease the motor current so that the operating before. So far as theoperator is concerned, the

functioning of the speed limit circuit and an indication on theoverspeed device I08 should be taken as a signal for releasing theaccelerator pedal to some extent thereby to reduce the power output orthe engine and to permit the system operating point to move oi! oi thecurrent limit. This is desirable, as obviously it is more eillcient tooperate the system at low current and high voltage than to operate atlow voltage and high current.

One outstanding feature of the motor control system described here byway oi illustration is that in'the event that the motor vehicle, whilemotoring forward, encounters a downhill grade and the operator makes noattempt to check the vehicle speed, the speed limit protection willoperate not only to reduce the engine horsepower output in the mannerpreviously explained, but will actually reverse the net excitation oithe traction motor field windings thereby automatically to initiatedynamic braking operation and to maintain the vehicle at the maximumpermissible speed. To illustrate. let it be assumed that the vehicle isbrought to a steep downhill grade with the engine Ill running at itsmaximum speed and the steering levers l3 and 84 pushed forward to theirmaximum torque positions, as

shown at Fig. 1. As the vehicle speed increases,

the voltage of the tachometer generator 60 will become so great that thecurrent spilled over from the tachometer generator through the battery4i and the motor excited diilerential windlugs 80 and GI will becomesufilcient to completely overpower the main control field windlugs 12and II to reverse the net excitation of the motor exciter l4 and ii atthe motors l6 and i1 thereby to initiate dynamic braking operation.

The manner in which the vehicle may be steered by reducing or reversingthe torque on one or the other of the motors has been previouslydescribed. Also, dynamic braking without steering by simultaneoustransfer of the steering handles l3 and 84 to their reverse quadrantshas been explained with reference to the motor field excitationcircuits. It may now be noted that when such dynamic braking operationis initiated, the back electr'omotive Some of the traction motors It andi1 is reduced to zero and then reversed. As the back electromotive forceof the traction motors approaches zero, or begins to reverse in goinginto braking, the directional coil II! or the reverse power relay RPRfunctions to drop out the reverse power relay thereby to deenergize theactuating coil "5,0! the braking contactor B and to cause the contactorB to drop out and to unshunt the braking resistor Ii. During dynamicbraking operation, the power generated by the generator I! and thetraction motors II and I1 is absorbed in the braking resistor. Properoperation or the reverse power relay RPR is ensured by a pair ofcapacitors I50 and Ill connected across the resistor I52 in the circuitof the directional coil 431. These capacitors function to cause thereverse power relay to anticipate its normal setting suiliciently sothat even when the rate of change of motor voltage is greatest thebraking contactor B is opened at the desired time. As braking proceedswith the motor voltage reversed. the reverse power relay RPR remainsdropped out and the braking contactor 3 likewise remains deenergized. Asthe motor voltage decreases with decreasing vehicle speed, the voltage01' the main generator I! increases to maintain the continuingunidirectional current in the main line circuit. Since the current inboth the motor armatures and the motor field windings is maintained.dynamic braking may be made eilective until the vehicle reachesstandstill.

Opening of the braking contactor B indicating that dynamic braking is inprogress also effects a number of protective functions necessitated bybraking operation. In the first place, the interlock contact 58 o! thecontactor B disables the current limit circuit thereby to remove thecurrent limit eflect during dynamic braking. This will be evident fromFig. 2 and is necessitated by the fact that the voltage of the enginedriven generator I! cannot be permitted to reverse under the influenceof the diflerentiai exciter field winding lll in an attempt to maintainthe line current at the current limit value. It the generator voltagewere permitted'to reverse, it would mean that the generator was actingas a motor to drive the internal combustion engine II.

While slight overspeeding of the engine "I is permissible, anysubstantial overspeeding must be avoided in order to prevent damage tothe engine. A second normally closed interlock Ill is also included onthe braking contactor B for the same reason. -When the braking contactorB drops out, the interlock It: shunts the rheostat it to ensure thatmaximum forward field is maintained on the generator exciter l3. Unlessthe generator exciter forward field ll is kept at its maximum valueduring braking, it is possible that even the minimum strength of thegenerator differential field It would be sufllcient to cause reversal ofthe main generator voltage and overspeeding of the engine It.

It is also to be noted that during braking the interlock contact ill onthe contactor B is closed to shunt the holding resistor I" in circuitwith the reverse power relay polarizing coil 13B thereby to change thecalibration of the reverse power relay and to restore its normal pick upcircuit.

The interlock contact I on the braking contactor B also closes duringdynamic braking to short circuit the holding resistor ill in series withthe actuating coil iii of the braking contactor and to restore thenormal pick up calibration oi the braking contactor.

During dynamic braking operation while the reverse power relay RPRremains dropped out, energization oi the braking contactor coil I35 isprecluded by the time delay relay TDR, the actuating winding I55 ofwhich is maintained energized through a. normally closed interlockcontact I58 on the reverse power relay RPR which shunts the now openinterlock contact 19 on the contactor B; when dynamic braking iscompleted and the vehicle speed is reduced to zero, the RPR relayrecognizes this fact by the fact that the motor voltage is substantiallyzero. The reverse power relay then picks up and deepergizes the forwardfield winding ll of the generator exciter It at the RPR interlockcontact I56. Opening of the contact I56 also deenersizes the time delayrelay TDR. The time delay relay, however, does not immediately drop out.Thus, for a short time, the generator exciter I3 is energized only byits differential field winding ill and its voltage is rapidly brought tozero. After a suitable time, which is just enough to allow the linecurrent to come approximately to zero, the time delay relay TDR dropsout and closes its interlock contact I42. Closure of the contact M2completes an energizing circuit for the braking 'contactor B and thecontactor B picks up and locks itself in through its interlock contactI. It will be understood, of course, that at this time the operator willrelease the dead man handle interlocks 53 and 54 and return the steeringhandles It and 84 to their normal forward biased positions. If theoperator retains the handles in the braking position. the reenergizationof the braking contactor B will complete an energizing circuit for thegenerator forward field winding through its interlock contact 9 andinitiate reverse motoring operation;

Referring now to Fig. 4, the purpose of a blocking rectifier lit, notpreviously mentioned, will become evident. The rectifier I" is locatedbetween the points 80 and 8| of the potentiometer bridges I4 and I5 andground and has for its purpose the prevention of a reverse current inthe main line circuit. Let it be assumed, for example, that the steeringhandles 83 and 84 are moved to braking position with the "dead manaso'aeio handle interlocks 5-3 and it open. Under these conditions, theforward field Ill of the-generator exclter It would be disabled so thatthere would be nothing to determine the direction of current flow in thegenerator output circuit. A reverse line current can start in the systemand, under. such circumstances, the reverse power relay would pick upunder the influence of the winding lll alone and cause closure of thebraking contactor B. Under such conditions, the traction motor 16 and Hwould build up as series generators as desired, but without the brakingresistor 2| in the main line circuit so that all the power would beabsorbed in the generator l2 thereby causing the generator to overspeedthe engine "I. The blocking rectifier I60 serves to prevent the motorsfrom receiving any excitation due to a reverse line current so thatbraking under such conditions is not possible. Thus, at Fig. 4, it willbe evident that if the potential of the grounded wire exceeds thepotential at the point 8|, as due to a reversal of line current, thepotentiometers I4 and It will be short circuited through the blockingrectifier I 69.

From the foregoing description, it will be evident that I have provideda generator excitation and engine governing system for'a gas-electricpower system which provides substantially immediate response of therelatively large amount of powerin the main generator field circuit bygoverning control of very small amounts of control power, preventsovershooting of the control point by eliminating "hunting" in thegenerator exciter operation, and permits maximum utilization ofavailable engine power over a very wide range of generator loadcurrents. While this governing apparatus is especially suitable forapplication to gas-electric propulsion systems for selfpropelledvehicles, it will be understood that it is not thus limited in itsapplication.

Furthermore, while I have described only one preferred embodiment of myinvention by way of illustration it will be evident that manymodifications will occur to those skilled in the art and that I,therefore, intend by the appended claims to cover all such modificationsas. fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent 0! the UnitedStates is:

1. A power system comprising a prime mover, a throttle valve for saidprime mover, a direct current generator driven by said prime mover andarranged to supply power to an electrical load, means'i'or controllingthe field excitation ot said generator, means responsive to rate ofchange of generator field excitation for opPO E any such change, and aspeed governor movable in response to the speed of said prime moversuccessively to control said throttle valve and said excitationcontrolling means thereby to maintain the power demand of said generatorsubstantially equal to the maximum available power output of said primemover throughout a wide range of generator load operation.

2. A power system comprising a prime mover, 'a throttle valve for saidprime mover, a speed governor for controlling said throttle valve, adirect current generator connected to be driven by said prime mover andarranged to s pply current to an electric motor, means for exciting saigenerator differentially in accordance with generator load current,means for changing the standard of excitation or said generator, meansresponsive to any change in generator excitation for opposing suchchange, and means controlled power output or said engine and by saidgovernor independently oi said throttle valve for controlling saidexcitation changing means to maintain the power demand of said generatorsubstantially equal to the maximum available power output of said primemover over a wide range of generator load operation.

3. A power system comprising a prime mover, a direct current generatordriven by said prime mover and arranged to supply current to an electricmotor, the inherent power demand 01' said generator being greater thanthe maximum available power output of said prime mover over asubstantial range of generator load operation but less than the maximumavailable power output oi said prime mover above and below said range,means for controlling the power output of said prime mover, variablefield excitation means for controlling the power demand of saidgenerator, means responsive to rate or change of generator fieldexcitation for opposing any such change, and means responsive to thespeed of said prime mover for sequentially actuating said prime moverand generator power controlling means. said speed responsive means beingarranged first to increase the power demand of said prime mover to itsmaximum value and thereafter to of said generator to 4. A power systemcomprising an internal combustion engine, a throttle valve for saidengine. a direct current generator driven by said engine and arranged tosupply power to an electric motor. an exciter for supplying excitationcurrent to said generator to render the inherent power demand 01' saidgenerator greater than the maximum available power output 01' saidengine over a substantial range of generator load operation but lessthan the maximum available a and below said range, means including acapacitor responsive to the rate of change oi exclter voltage foropposing changes in generator ileld excitation. and a speed governormovable in response to the speed control said throttle valve and saidgenerator excitation thereby to maintain the power demand of saidgenerator substantially equal to the maximum available power output orsaid engine throughout said range 01' operation,

5. A power system comprising a substantially constant speed internalcombustion engine, a throttle for said ngine movable between idling andfull open positions, a direct current generator driven by said engineand arranged to supply power to an electric motor, an armature reactionexcited direct current exciter f r supplying field excitation current tosaid generator, a variable reference field exciting winding for saidexciter, a difl'erential field exciting winding for said exclterenergized in accordance with main generator load current, a capacitorcontrolled ileld exciting winding for said exciter transiently energizedin accordance with rate of change of exciter voltage to oppose any suchchange. a speed governor for controlling said throttle valve. saidgovernor being provided with a predetermined amount of over-travel, andmeans responsive to over-travel 01' said governor occasioned byinability of said engine to maintain its governed speed with saidthrottle in said full open position for controlling the energlzation ofsaid reference field exciting winding to decrease the power demand ofsaid generator to equal the maximum available power output-ot saidengine.

6. An electric power system comprising a prime mover, a throttle valvefor said prime mover, a direct current generator driven by said primemover and arranged to supply power to an electrical load, an armaturereaction excited dynamoelectric exciting machine ior supplying fieldexcitation current to said generator, means responslve to the rate ofchange of output voltage of said dynamoelectric exciting machine foropposing any such change. and a speed governor movable in response tothe speed of said prime mover successively to control said throttlevalve and said output voltage of said dynamoelectrlc exciting machinethereby to maintain the power demand of said generator substantially eqal t the maximum available power output of said prime mover throughout awide range of generator load operation.

7. An electric power system comprising a prime mover, a throttle valvefor said prime mover, a direct current generator driven by said primemover and arranged to supply current to an electrical load, an armaturereaction excited dynamoelectric exciting machine for supplyingexcitation current to said generator, means for controlling theexcitation of said dynamoelectric excitin machine in accordance with thecurrent supplied to said electrical load, means responsive to the rateor change of output voltage of said exciting machine for opposing anysuch change, and a speed governor movable in response to th sp ed 01'said prime mover succeuively to control said throttle valve and thestandard of excitation of said exciting machine thereby to maintain thepower demand of said generator substantially equal to the maximumavailable power output of said prime mover throughout a wide range 01enerator load operation.

MARTIN A. EDWARDS.

Certificate of Correction Patent No. 2,393,619. January 29, 1946.

MARTIN A. EDWARDS It is hereby certified that errors appear in theprinted specification of t e above numbered patent requiring correctionas follows: Page 6, second column, lines 59 and 60, strike out "Themaximum vehicle speed is indicated at Fig. 6 by a line 101.; page 9,second column, line 27, for occurrence read occurrence; page 10, secondcolumn, line 19, for FPR read RPR; line 24, strike out "for an actuatingcoil 135 of the bra and insert instead is provided with a polarizingwinding; line 30, forwinding 146 read winding 136; page 12, secondcolumn, line 11, for motor" read motors; and that the said LettersPatent should be read with these corrections therein that the same mayconform to the record of the case in the Patent Office.

Signed and sealed this 13th day of August, A. D. 1946.

LESLIE FRAZER,

First Assistant Commissioner of Patents.

